3. Lithium Through Neon   Previous PageNext Page
     How Big is an Atom?

A helium atom is smaller than a hydrogen atom. Although helium has one more electron, it also has an additional positive charge on the nucleus that pulls both electrons closer to the nucleus.

Atoms of the second-shell elements Li through Ne are larger than either hydrogen or helium because the second electron shell is farther from the nucleus. Also, there is a gradual decrease in atomic size from Li to Ne. The inner two electrons shield the nucleus, so the outer electrons experience the pull from a "helium core" with a net positive charge that is two less than the actual atomic number.

The lone outer electron in Li feels a net central charge of +3 - 2 = +1. The four outer electrons in C experience a central attraction of +6 - 2 = +4, and the seven electrons around F experience a charge attraction of +7. As the central charge increases with atomic number from Li to Ne, the outer electrons are drawn more tightly toward the nucleus by electrostatic attraction, thereby causing the atoms to be successively smaller. This has an important effect on their chemical behavior.

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